Magnetic tape having characterized magnetic layer and magnetic recording and reproducing device

ABSTRACT

The magnetic tape includes a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which an absolute value ΔN of a difference between a refractive index Nxy measured regarding an in-plane direction of the magnetic layer and a refractive index Nz measured regarding a thickness direction of the magnetic layer is 0.25 to 0.40, and a logarithmic decrement acquired by a pendulum viscoelasticity test performed regarding a surface of the magnetic layer is equal to or smaller than 0.050.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 16/361,597filed Mar. 22, 2019 which claims priority under 35 U.S.C. 119 toJapanese Patent Application No. 2018-057168 filed on Mar. 23, 2018 andJapanese Patent Application No. 2019-050217 filed on Mar. 18, 2019. Eachof the above applications is hereby expressly incorporated by reference,in its entirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape and a magneticrecording and reproducing device.

2. Description of the Related Art

Magnetic recording media are divided into tape-shaped magnetic recordingmedia and disk-shaped magnetic recording media, and tape-shaped magneticrecording media, that is, magnetic tapes are mainly used for datastorage. The recording of information on a magnetic tape and/orreproducing thereof are normally performed by causing a surface of themagnetic tape (surface of a magnetic layer) to come into contact with amagnetic head (hereinafter, also simply referred to as a head”) forsliding. As the magnetic tape, a magnetic tape having a configuration inwhich a magnetic layer including ferromagnetic powder and a bindingagent is provided on a non-magnetic support is widely used (for example,see JP2005-243162A).

SUMMARY OF THE INVENTION

In a case of reproducing information recorded on a magnetic tape, as afrequency of generation of a partial decrease in reproducing signalamplitude (referred to as “missing pulse”) increases, an error rateincreases and reliability of the magnetic tape decreases. Therefore, inorder to provide a magnetic tape capable of being used with highreliability, it is desired to decrease a generation frequency of themissing pulse.

However, in recent years, the magnetic tape used for data storage isused in a data center in which a temperature and humidity are managed.On the other hand, in the data center, power saving is necessary forreducing the cost. For realizing the power saving, the managingconditions of the temperature and humidity of the data center can bealleviated compared to the current state, or the managing may not benecessary. However, in a case where the managing conditions of thetemperature and humidity are alleviated or the managing is notperformed, the magnetic tape is assumed to be used in variousenvironments and assumed to be used in an environment of a lowtemperature and high humidity. However, as a result of the studies ofthe inventors, in a low temperature and high humidity environment, ithas been determined that a frequency of generation of the missing pulsetends to increase.

Therefore, an object of the invention is to provide a magnetic tape inwhich a generation frequency of a missing pulse in the low temperatureand high humidity environment is decreased.

According to one aspect of the invention, there is provided a magnetictape comprising: a non-magnetic support; and a magnetic layer includingferromagnetic powder and a binding agent on the non-magnetic support, inwhich an absolute value ΔN of a difference between a refractive indexNxy measured regarding an in-plane direction of the magnetic layer and arefractive index Nz measured regarding a thickness direction of themagnetic layer is 0.25 to 0.40, and a logarithmic decrement acquired bya pendulum viscoelasticity test performed regarding a surface of themagnetic layer (hereinafter, also referred to as a “logarithmicdecrement of the magnetic layer” or simply a “logarithmic decrement”) isequal to or smaller than 0.050.

In one aspect, the difference (Nxy−Nz) between the refractive index Nxyand the refractive index Nz may be 0.25 to 0.40.

In one aspect, the logarithmic decrement may be 0.010 to 0.050.

In one aspect, the magnetic tape may further comprise a non-magneticlayer including a non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.

In one aspect, the magnetic tape may further comprise a back coatinglayer including a non-magnetic powder and a binding agent on a surfaceof the non-magnetic support opposite to a surface provided with themagnetic layer.

According to another aspect of the invention, there is provided amagnetic recording and reproducing device comprising: the magnetic tape;and a magnetic head.

According to one aspect of the invention, it is possible to provide amagnetic tape in which a generation frequency of a missing pulse in thelow temperature and high humidity environment is decreased. In addition,according to the other aspect of the invention, it is possible toprovide a magnetic recording and reproducing device including themagnetic tape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram of a measurement method of alogarithmic decrement.

FIG. 2 is an explanatory diagram of the measurement method of thelogarithmic decrement.

FIG. 3 is an explanatory diagram of the measurement method of thelogarithmic decrement.

FIG. 4 shows an example (schematic step view) of a specific aspect of amagnetic tape manufacturing step.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Tape

One aspect of the invention relates to a magnetic tape including: anon-magnetic support; and a magnetic layer including ferromagneticpowder and a binding agent on the non-magnetic support, in which anabsolute value ΔN of a difference between a refractive index Nxymeasured regarding an in-plane direction of the magnetic layer and arefractive index Nz measured regarding a thickness direction of themagnetic layer is 0.25 to 0.40, and a logarithmic decrement acquired bya pendulum viscoelasticity test performed regarding a surface of themagnetic layer is equal to or smaller than 0.050.

In the invention and the specification, the “surface of the magneticlayer” is identical to a surface of a magnetic tape on the magneticlayer side. In the invention and the specification, the “ferromagneticpowder” means an aggregate of a plurality of ferromagnetic particles.The “aggregate” is not only limited to an aspect in which particlesconfiguring the aggregate directly come into contact with each other,but also includes an aspect in which a binding agent, an additive, orthe like is interposed between the particles. The points described aboveare also applied to various powders such as non-magnetic powder of theinvention and the specification, in the same manner.

Hereinafter, measurement methods of ΔN and the logarithmic decrementwill be described.

In the invention and the specification, the absolute value ΔN of thedifference between the refractive index Nxy measured regarding thein-plane direction of the magnetic layer and the refractive index Nzmeasured regarding the thickness direction of the magnetic layer is avalue obtained by the following method.

The refractive index regarding each direction of the magnetic layer isobtained using a double-layer model by spectral ellipsometry. In orderto obtain the refractive index of the magnetic layer using thedouble-layer model by spectral ellipsometry, a value of a refractiveindex of a portion adjacent to the magnetic layer is used. Hereinafter,an example in a case of obtaining the refractive indexes Nxy and Nz ofthe magnetic layer of the magnetic tape including a layer configurationin which the non-magnetic layer and the magnetic layer are laminated onthe non-magnetic support in this order will be described. However, themagnetic tape according to one aspect of the invention may also be amagnetic tape having a layer configuration in which the magnetic layeris directly laminated on the non-magnetic support without thenon-magnetic layer interposed therebetween. Regarding the magnetic tapehaving such a configuration, the refractive index regarding eachdirection of the magnetic layer is obtained in the same manner as thefollowing method, using the double-layer model of the magnetic layer andthe non-magnetic support. In addition, an incidence angle shown below isan incidence angle in a case where the incidence angle is 0° in a caseof vertical incidence.

(1) Preparation of Sample for Measurement

Regarding the magnetic tape including a back coating layer on a surfaceof a non-magnetic support on a side opposite to the surface providedwith a magnetic layer, the measurement is performed after removing theback coating layer of a sample for measurement cut from the magnetictape. The removal of the back coating layer can be performed by awell-known method of dissolving of the back coating layer using asolvent or the like. As the solvent, for example, methyl ethyl ketonecan be used. However, any solvent which can remove the back coatinglayer may be used. The surface of the non-magnetic support afterremoving the back coating layer is roughened by a well-known method sothat the reflected light on this surface is not detected, in themeasurement of ellipsometer. The roughening can be performed by a methodof polishing the surface of the non-magnetic support after removing theback coating layer by using sand paper, for example. Regarding thesample for measurement cut out from the magnetic tape not including theback coating layer, the surface of the non-magnetic support on a sideopposite to the surface provided with the magnetic layer is roughened.

In addition, in order to measure the refractive index of thenon-magnetic layer described below, the magnetic layer is furtherremoved and the surface of the non-magnetic layer is exposed. In orderto measure the refractive index of the non-magnetic support describedbelow, the non-magnetic layer is also further removed and the surface ofthe non-magnetic support on the magnetic layer side is exposed. Theremoval of each layer can be performed by a well-known method so asdescribed regarding the removal of the back coating layer. Alongitudinal direction described below is a direction which was alongitudinal direction of the magnetic tape, in a case where the samplefor measurement is included in the magnetic tape before being cut out.This point applies to other directions described below, in the samemanner.

(2) Measurement of Refractive Index of Magnetic Layer

By setting the incidence angles as 65°, 70°, and 75°, and irradiatingthe surface of the magnetic layer in the longitudinal direction with anincidence ray having a beam diameter of 300 μm by using theellipsometer, Δ (phase difference of s-polarized light and p-polarizedlight) and ψ (amplitude ratio of s-polarized light and p-polarizedlight) is measured. The measurement is performed by changing awavelength of the incidence ray by 1.5 nm in a range of 400 to 700 nm,and a measurement value at each wavelength is obtained.

The refractive index of the magnetic layer at each wavelength isobtained with a double-layer model as described below, by using themeasurement values of Δ and ψ of the magnetic layer at each wavelength,the refractive index of the non-magnetic layer in each directionobtained by the following method, and the thickness of the magneticlayer.

The zeroth layer which is a substrate of the double-layer model is setas a non-magnetic layer and the first layer thereof is set as a magneticlayer. The double-layer model is created by assuming that there is noeffect of rear surface reflection of the non-magnetic layer, by onlyconsidering the reflection of the interfaces of air/magnetic layer andmagnetic layer/non-magnetic layer. A refractive index of the first layerwhich is fit to the obtained measurement value the most is obtained byfitting performed by a least squares method. The refractive index Nx ofthe magnetic layer in the longitudinal direction and a refractive indexNz₁ of the magnetic layer in the thickness direction measured byemitting the incidence ray in the longitudinal direction are obtained asvalues at the wavelength of 600 nm obtained from the results of thefitting.

In the same manner as described above, except that the direction ofincidence of the incidence ray is set as a width direction of themagnetic tape, a refractive index Ny of the magnetic layer in the widthdirection and a refractive index Nz₂ of the magnetic layer in thethickness direction measured by emitting the incidence ray in the widthdirection are obtained as values at the wavelength of 600 nm obtainedfrom the results of the fitting.

The fitting is performed by the following method.

In general, “complex refractive index n=η+iκ”. Here, η is a real numberof the refractive index, κ is an extinction coefficient, and i is animaginary number. In a case where a complex dielectric constantε=ε1+iε2(ε1 and ε2 satisfies Kramers-Kronig relation), ε1=η²−κ², andε2=2ηκ, the complex dielectric constant of Nx satisfies thatε_(x)=ε_(x)1+iε_(x)2, and the complex dielectric constant of Nz₁satisfies that ε_(z1)=ε_(z1)1+iε_(z1)2, in a case of calculating the Nxand Nz₁.

The Nx is obtained by setting ε_(x)2 as one Gaussian, setting any point,where a peak position is 5.8 to 5.1 eV and σ is 4 to 3.5 eV, as astarting point, setting a parameter to be offset to a dielectricconstant beyond a measurement wavelength range (400 to 700 nm), andperforming least squares fitting with respect to the measurement value.In the same manner, N_(z)1 is obtained by setting any point of ε_(z1)2,where a peak position is 3.2 to 2.9 eV and σ is 1.5 to 1.2 eV, as astarting point, and setting an offset parameter, and performing leastsquares fitting with respect to the measurement value. Ny and Nz₂ arealso obtained in the same manner. The refractive index Nxy measuredregarding the in-plane direction of the magnetic layer is obtained as“Nxy=(Nx+Ny)/2”. The refractive index Nz measured regarding thethickness direction of the magnetic layer is obtained as“Nz=(Nz₁+Nz₂)/2”. From the obtained Nxy and Nz, the absolute value ΔN ofdifference thereof is obtained.

(3) Measurement of Refractive Index of Non-Magnetic Layer

Refractive indexes of the non-magnetic layer at a wavelength of 600 nm(the refractive index in the longitudinal direction, the refractiveindex in the width direction, the refractive index in the thicknessdirection measured by emitting the incidence ray in the longitudinaldirection, and the refractive index in the thickness direction measuredby emitting the incidence ray in the width direction) are obtained inthe same manner as in the method described above, except the followingpoints.

The wavelength of the incidence ray is changed by 1.5 nm in the range of250 to 700 nm.

By using a double-layer model of a non-magnetic layer and a non-magneticsupport, the zeroth layer which is a substrate of the double-layer modelis set as the non-magnetic support, and the first layer thereof is setas the non-magnetic layer. The double-layer model is created by assumingthat there is no effect of rear surface reflection of the non-magneticsupport, by only considering the reflection of the interfaces ofair/non-magnetic layer and non-magnetic layer/non-magnetic support.

In the fitting, seven peaks (0.6 eV, 2.3 eV, 2.9 eV, 3.6 eV, 4.6 eV, 5.0eV, and 6.0 eV) are assumed in the imaginary part (ε2) of the complexdielectric constant, and the parameter to be offset is set to thedielectric constant beyond the measurement wavelength range (250 to 700nm).

(4) Measurement of Refractive Index of Non-Magnetic Support

The refractive indexes of the non-magnetic support at a wavelength of600 nm (refractive index in the longitudinal direction, the refractiveindex in the width direction, the refractive index in the thicknessdirection measured by emitting the incidence ray in the longitudinaldirection, and the refractive index in the thickness direction measuredby emitting the incidence ray in the width direction) used for obtainingthe refractive indexes of the non-magnetic layer by the double-layermodel are obtained in the same manner as in the method described abovefor measuring the refractive index of the magnetic layer, except thefollowing points.

A single-layer model with only front surface reflection is used, withoutusing the double-layer model.

The fitting is performed by the Cauchy model (n=A+B/λ², n is arefractive index, A and B are respectively constants determined byfitting, and λ is a wavelength).

Next, the logarithmic decrement of the magnetic layer will be described.

In the invention and the specification, the logarithmic decrement of themagnetic layer is a value acquired by the following method.

FIGS. 1 to 3 are explanatory diagrams of a measurement method of thelogarithmic decrement. Hereinafter, the measurement method of thelogarithmic decrement will be described with reference to the drawings.However, the aspect shown in the drawing is merely an example and theinvention is not limited thereto.

A measurement sample 100 is cut out from the magnetic tape which is ameasurement target. The cut-out measurement sample 100 is placed on asubstrate 103 so that a measurement surface (surface of the magneticlayer) faces upward, in a sample stage 101 in a pendulum viscoelasticitytester, and the measurement sample is fixed by fixing tapes 105 in astate where obvious wrinkles which can be visually confirmed are notgenerated.

A pendulum-attached columnar cylinder edge 104 (diameter of 4 mm) havingmass of 13 g is loaded on the measurement surface of the measurementsample 100 so that a long axis direction of the cylinder edge becomesparallel to a longitudinal direction of the measurement sample 100. Anexample of a state in which the pendulum-attached columnar cylinder edge104 is loaded on the measurement surface of the measurement sample 100as described above (state seen from the top) is shown in FIG. 1 . In theaspect shown in FIG. 1 , a holder and temperature sensor 102 isinstalled and a temperature of the surface of the substrate 103 can bemonitored. However, this configuration is not essential. In the aspectshown in FIG. 1 , the longitudinal direction of the measurement sample100 is a direction shown with an arrow in the drawing, and is the samedirection as a longitudinal direction of a magnetic tape from which themeasurement sample is cut out. In addition, as a pendulum 107 (see FIG.2 ), a pendulum formed of a material having properties of being adsorbedto a magnet (for example, formed of metal or formed of an alloy) isused.

The temperature of the surface of the substrate 103 on which themeasurement sample 100 is placed is set to 80° C. by increasing thetemperature at a rate of temperature increase equal to or lower than 5°C./min (any rate of temperature increase may be set, as long as it isequal to or lower than 5° C./min), and the pendulum movement is started(induce initial vibration) by releasing adsorption between the pendulum107 and a magnet 106. An example of a state of the pendulum 107 whichperforms the pendulum movement (state seen from the side) is shown inFIG. 2 . In the aspect shown in FIG. 2 , in the pendulum viscoelasticitytester, the pendulum movement is started by stopping (switching off) theelectricity to the magnet (electromagnet) 106 disposed on the lower sideof the sample stage to release the adsorption, and the pendulum movementis stopped by restarting (switching on) the electricity to theelectromagnet to cause the pendulum 107 to be adsorbed to the magnet106. As shown in FIG. 2 , during the pendulum movement, the pendulum 107reciprocates the amplitude. From a result obtained by monitoringdisplacement of the pendulum with a displacement sensor 108 while thependulum reciprocates the amplitude, a displacement-time curve in whicha vertical axis indicates the displacement and a horizontal axisindicates the elapsed time is obtained. An example of thedisplacement-time curve is shown in FIG. 3 . FIG. 3 schematically showscorrespondence between the state of the pendulum 107 and thedisplacement-time curve. The stop (adsorption) and the pendulum movementare repeated at a regular measurement interval, the logarithmicdecrement Δ (no unit) is acquired from the following Expression by usinga displacement-time curve obtained in the measurement interval after 10minutes or longer (may be any time, as long as it is 10 minutes orlonger) has elapsed, and this value is set as logarithmic decrement ofthe surface of the magnetic layer of the magnetic tape. The adsorptiontime of the first adsorption is set as 1 second or longer (may be anytime, as long as it is 1 second or longer), and the interval between theadsorption stop and the adsorption start is set as 6 seconds or longer(may be any time, as long as it is 6 seconds or longer). The measurementinterval is an interval of the time from the adsorption start and thenext adsorption start. In addition, humidity of an environment in whichthe pendulum movement is performed, may be any relative humidity, aslong as the relative humidity is 40% to 70%.

$\Delta = \frac{{\ln( \frac{A_{1}}{A_{2}} )} + {\ln( \frac{A_{2}}{A_{3}} )} + {\ldots\mspace{14mu}{\ln( \frac{A_{n}}{A_{n + 1}} )}}}{n}$

In the displacement-time curve, an interval between a point of theminimum displacement and a point of the next minimum displacement is setas a period of a wave. n indicates the number of waves included in thedisplacement-time curve in the measurement interval, and A_(n) indicatesthe minimum displacement and maximum displacement of the n-th wave. InFIG. 3 , an interval between the minimum displacement of the n-th waveand the next minimum displacement is shown as P_(n) (for example, P₁regarding the first wave, P₂ regarding the second wave, and P₃ regardingthe third wave). In the calculation of the logarithmic decrement, adifference (in Expression A_(n+1), in the displacement-time curve shownin FIG. 3 , A₄) between the minimum displacement and the maximumdisplacement appearing after the n-th wave is also used, but a partwhere the pendulum 107 stops (adsorption) after the maximum displacementis not used in the counting of the number of waves. In addition, a partwhere the pendulum 107 stops (adsorption) before the maximumdisplacement is not used in the counting of the number of waves, either.Accordingly, the number of waves is 3 (n=3) in the displacement-timecurve shown in FIG. 3 .

The inventors have surmised as follows regarding a reason for a decreaseof the generation frequency of the missing pulse in the low temperatureand high humidity environment in the magnetic tape.

In a case of reproducing information recorded on the magnetic tape, in acase where the surface of the magnetic layer is chipped in the slidingof the surface of the magnetic layer and a head, the generated scrapsare attached to the head and a head attached material may be generated.The inventors have surmised regarding the reason for the generation ofthe missing pulse in the low temperature and high humidity environment,a contact state in a case of the sliding of the surface of the magneticlayer and the head easily becomes unstable due to a tendency of anincrease of a coefficient of friction during the sliding of the surfaceof the magnetic layer and the head in the low temperature and highhumidity environment, and the reason for the unstable contact state isthe generation of the head attached material.

Regarding the above-mentioned point, the inventors have thought that ΔNobtained by the method described above is a value which may be an indexof a presence state of the ferromagnetic powder in a surface region ofthe magnetic layer. This ΔN is assumed as a value which is influenced bythe effect of various factors such as a presence state of a bindingagent or a density distribution of the ferromagnetic powder, in additionto the alignment state of the ferromagnetic powder in the magneticlayer. In addition, it is thought that the magnetic layer in which theΔN is set as 0.25 to 0.40 by controlling various factors has a highhardness of the surface of the magnetic layer and the chipping thereofdue to the sliding with the head hardly occurs. The inventors havesurmised that, this contributes to the prevention of the generation ofthe head attached material due to the chipping of the surface of themagnetic layer during the sliding with the head in the low temperatureand high humidity environment, and as a result, this contributes to adecrease in the generation frequency of the missing pulse in the lowtemperature and high humidity environment.

In addition, the inventors have surmised regarding the logarithmicdecrement as follows.

It is thought that the logarithmic decrement obtained by the methoddescribed above is a value which is an index for the amount of pressuresensitive adhesive components separated from the surface of the magneticlayer, in a case where the head comes into contact with the surface ofthe magnetic layer and slides thereon, and interposed between thesurface of the magnetic layer and the head. It is thought that as alarger amount of the pressure sensitive adhesive components are present,adhesion between the surface of the magnetic layer and the headincreases, and accordingly, a contact state during the sliding betweenthe surface of the magnetic layer and the head becomes unstable. Withrespect to this, it is thought that, in the magnetic tape, a state wherethe logarithmic decrement of the magnetic layer is equal to or smallerthan 0.050, that is, a state where the amount of the pressure sensitiveadhesive components is decreased contributes to smooth sliding betweenthe surface of the magnetic layer and the head. The inventors havesurmised that this contributes to stabilization of the contact stateduring the sliding between the surface of the magnetic layer and thehead, thereby causing a decrease in generation frequency of the missingpulse in the low temperature and high humidity environment.

The details of the pressure sensitive adhesive components are not clear.The inventors have surmised that the pressure sensitive adhesivecomponents may be derived from a resin used as a binding agent. Thespecific description is as follows. As a binding agent, various resinscan be used as will be described later in detail. The resin is a polymer(including a homopolymer or a copolymer) of two or more polymerizablecompounds and generally also includes a component having a molecularweight which is smaller than an average molecular weight (hereinafter,referred to as a “binding agent component having a low molecularweight”). The inventors have thought that the binding agent componenthaving a low molecular weight may be separated from the surface of themagnetic layer at the time of sliding between the head and the surfaceof the magnetic layer and interposed between the surface of the magneticlayer and the head. The inventors have surmised that, the binding agentcomponent having a low molecular weight may have pressure sensitiveadhesive properties and the logarithmic decrement acquired by a pendulumviscoelasticity test may be an index for the amount of binding agentcomponents having a low molecular weight separated from the surface ofthe magnetic layer at the time of the sliding between the surface of themagnetic layer and the head. In one aspect, the magnetic layer is formedby applying a magnetic layer forming composition including a curingagent in addition to ferromagnetic powder and a binding agent, onto anon-magnetic support directly or with another layer interposedtherebetween, and performing curing process. With the curing processhere, it is possible to allow a curing reaction (crosslinking reaction)between the binding agent and the curing agent. However, although thereason thereof is not clear, the inventors have considered that thebinding agent component having a low molecular weight may have poorreactivity regarding the curing reaction. Accordingly, the inventorshave surmised that the binding agent component having a low molecularweight which hardly remains in the magnetic layer and is easilyseparated from the magnetic layer may be one of reasons for that thebinding agent component having a low molecular weight is interposedbetween the surface of the magnetic layer and the head at the time ofthe sliding between the surface of the magnetic layer and the head.

However, the above descriptions are merely a surmise of the inventorsand the invention is not limited thereto.

Hereinafter, the magnetic tape will be described more specifically.Hereinafter, the generation frequency of the missing pulse in the lowtemperature and high humidity environment is also simply referred to asthe “generation frequency of the missing pulse”.

Magnetic Layer

ΔN of Magnetic Layer

ΔN of the magnetic layer of the magnetic tape is 0.25 to 0.40. Asdescribed above, it is surmised that the magnetic layer having ΔN of0.25 to 0.40 has a high hardness of the surface of the magnetic layer,and the chipping thereof due to the sliding with the head in the lowtemperature and high humidity environment hardly occurs. Accordingly, itis thought that, in a case of reproducing information recorded on themagnetic layer in the low temperature and high humidity environment, thechipping of the magnetic layer having ΔN in the range described abovehardly occurs on the surface of the magnetic layer during the sliding ofthe surface of the magnetic layer and the head. It is surmised that thiscontributes to a decrease in the generation frequency of the missingpulse in the low temperature and high humidity environment. From aviewpoint of further decreasing the generation frequency of the missingpulse, ΔN is preferably 0.25 to 0.35. A specific aspect of means foradjusting ΔN will be described later.

ΔN is an absolute value of a difference between Nxy and Nz. Nxy is arefractive index measured regarding the in-plane direction of themagnetic layer and Nz is a refractive index measured regarding thethickness direction of the magnetic layer. In one aspect, a relation ofNxy>Nz can be satisfied, and in the other aspect, Nxy<Nz can besatisfied. From a viewpoint of electromagnetic conversioncharacteristics of the magnetic tape, a relationship of Nxy>Nz ispreferable, and therefore, the difference between the Nxy and Nz(Nxy−Nz) is preferably 0.25 to 0.40 and more preferably 0.25 to 0.35.

Various means for adjusting ΔN described above will be described later.

Logarithmic Decrement

The logarithmic decrement acquired by a pendulum viscoelasticity testperformed regarding the surface of the magnetic layer of the magnetictape is equal to or smaller than 0.050, from a viewpoint of decreasingthe generation frequency of the missing pulse in the low temperature andhigh humidity environment. From a viewpoint of further decreasing thegeneration frequency of the missing pulse, the logarithmic decrement ispreferably equal to or smaller than 0.048, more preferably equal to orsmaller than 0.045, and even more preferably equal to or smaller than0.040. Meanwhile, from a viewpoint of decreasing the generationfrequency of the missing pulse, it is preferable that the logarithmicdecrement is low, and thus, a lower limit value is not particularlylimited. The logarithmic decrement can be, for example, equal to orgreater than 0.010 or equal to or greater than 0.015. However, thelogarithmic decrement may be smaller than the exemplified value. Aspecific aspect of means for adjusting the logarithmic decrement will bedescribed later.

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer,ferromagnetic powder normally used in the magnetic layer of variousmagnetic recording media can be used. It is preferable to useferromagnetic powder having a small average particle size, from aviewpoint of improvement of recording density of the magnetic recordingmedium. From this viewpoint, ferromagnetic powder having an averageparticle size equal to or smaller than 50 nm is preferably used as theferromagnetic powder. Meanwhile, the average particle size of theferromagnetic powder is preferably equal to or greater than 5 nm, andmore preferably equal to or greater than 10 nm, from a viewpoint ofstability of magnetization.

As a preferred specific example of the ferromagnetic powder,ferromagnetic hexagonal ferrite powder can be used. The ferromagnetichexagonal ferrite powder can be ferromagnetic hexagonal barium ferritepowder, ferromagnetic hexagonal strontium ferrite powder, and the like.An average particle size of the ferromagnetic hexagonal ferrite powderis preferably 10 nm to 50 nm and more preferably 20 nm to 50 nm, from aviewpoint of improvement of recording density and stability ofmagnetization. For details of the ferromagnetic hexagonal ferritepowder, descriptions disclosed in paragraphs 0012 to 0030 ofJP2011-225417A, paragraphs 0134 to 0136 of JP2011-216149A, andparagraphs 0013 to 0030 of JP2012-204726A can be referred to, forexample.

As a preferred specific example of the ferromagnetic powder,ferromagnetic metal powder can also be used. An average particle size ofthe ferromagnetic metal powder is preferably 10 nm to 50 nm and morepreferably 20 nm to 50 nm, from a viewpoint of improvement of recordingdensity and stability of magnetization. For details of the ferromagneticmetal powder, descriptions disclosed in paragraphs 0137 to 0141 ofJP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A can bereferred to, for example.

As a preferred specific example of the ferromagnetic powder, ε-ironoxide powder can also be used. As a method for producing ε-iron oxidepowder, a method for producing ε-iron oxide powder from goethite and areverse micelle method has been known. Both of the above-describedproduction methods have been publicly known. Moreover, J. Jpn. Soc.Powder Metallurgy Vol. 61 Supplement, No. S1, pp. S280-S284 and J.Mater. Chem. C, 2013, 1, pp. 5200-5206 can be referred to about a methodfor producing ε-iron oxide powder where some of Fe are substituted withsubstitutional atoms such as Ga, Co, Ti, Al, and Rh, for example. Themethod for producing ε-iron oxide powder which can be used asferromagnetic powder in the magnetic layer, however, is not limited tothese methods.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder and the like are valuesmeasured by the following method with a transmission electronmicroscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, the image is printed on photographicprinting paper so that the total magnification ratio of 500,000 toobtain an image of particles configuring the powder. A target particleis selected from the obtained image of particles, an outline of theparticle is traced with a digitizer, and a size of the particle (primaryparticle) is measured. The primary particle is an independent particlewhich is not aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. A value regarding asize of powder such as the average particle size shown in examples whichwill be described later is a value measured by using transmissionelectron microscope H-9000 manufactured by Hitachi, Ltd. as thetransmission electron microscope, and image analysis software KS-400manufactured by Carl Zeiss as the image analysis software, unlessotherwise noted.

As a method of collecting a sample powder from the magnetic recordingmedium in order to measure the particle size, a method disclosed in aparagraph of 0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles configuring thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter. In a case of the definition (3), the averageparticle size is an average diameter (also referred to as an averageparticle diameter).

In one aspect, the shape of the ferromagnetic particles configuring theferromagnetic powder included in the magnetic layer can be a plateshape. Hereinafter, the ferromagnetic powder including the plate-shapedferromagnetic particles is referred to as a plate-shaped ferromagneticpowder. An average plate ratio of the plate-shaped ferromagnetic powdercan be preferably 2.5 to 5.0. The average plate ratio is an arithmeticalmean of (maximum long diameter/thickness or height) in a case of thedefinition (2). As the average plate ratio increases, uniformity of thealignment state of the ferromagnetic particles configuring theplate-shaped ferromagnetic powder tends to easily increase by thealignment process, and the value of ΔN tends to increase.

As an index for a particle size of the ferromagnetic powder, anactivation volume can also be used. The “activation volume” is a unit ofmagnetization reversal. Regarding the activation volume described in theinvention and the specification, magnetic field sweep rates of acoercivity Hc measurement part at time points of 3 minutes and 30minutes are measured by using an oscillation sample type magnetic-fluxmeter in an environment of an atmosphere temperature of 23° C.±1° C.,and the activation volume is a value acquired from the followingrelational expression of Hc and an activation volume V. The activationvolume shown in examples which will be described later is a valueobtained by performing measurement using an oscillation sample typemagnetic-flux meter (manufactured by Toei Industry Co., Ltd.).Hc=2Ku/Ms{1−[(kT/KuV)ln(At/0.693)]^(1/2)}

[In the expression, Ku: anisotropy constant, Ms: saturationmagnetization, k: Boltzmann's constant, T: absolute temperature, V:activation volume, A: spin precession frequency, and t: magnetic fieldreversal time]

From a viewpoint of improving the recording density, the activationvolume of the ferromagnetic powder is preferably equal to or smallerthan 2,500 nm³, more preferably equal to or smaller than 2,300 nm³, andeven more preferably equal to or smaller than 2,000 nm³. Meanwhile, froma viewpoint of stability of magnetization, the activation volume of theferromagnetic powder is, for example, preferably equal to or greaterthan 800 nm³, more preferably equal to or greater than 1,000 nm³, andeven more preferably equal to or greater than 1,200 nm³.

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably 50% to 90% by mass and more preferably 60%to 90% by mass. The components other than the ferromagnetic powder ofthe magnetic layer are at least one or more components selected from thegroup consisting of fatty acid and fatty acid amide, and a bindingagent, and one or more kinds of additives may be randomly included. Ahigh filling percentage of the ferromagnetic powder in the magneticlayer is preferable from a viewpoint of improvement of recordingdensity.

Binding Agent and Curing Agent

The magnetic tape is a coating type magnetic tape and includes a bindingagent in the magnetic layer. The binding agent is one or more kinds ofresin. The resin may be a homopolymer or a copolymer. As the bindingagent included in the magnetic layer, a resin selected from apolyurethane resin, a polyester resin, a polyamide resin, a vinylchloride resin, an acrylic resin obtained by copolymerizing styrene,acrylonitrile, or methyl methacrylate, a cellulose resin such asnitrocellulose, an epoxy resin, a phenoxy resin, and a polyvinylalkylalresin such as polyvinyl acetal or polyvinyl butyral can be used alone ora plurality of resins can be mixed with each other to be used. Amongthese, a polyurethane resin, an acrylic resin, a cellulose resin, and avinyl chloride resin are preferable. These resins can be used as thebinding agent even in the non-magnetic layer and/or a back coating layerwhich will be described later. For the binding agent described above,description disclosed in paragraphs 0029 to 0031 of JP2010-024113A canbe referred to. An average molecular weight of the resin used as thebinding agent can be, for example, 10,000 to 200,000 as a weight-averagemolecular weight. The weight-average molecular weight of the inventionand the specification is a value obtained by performing polystyreneconversion of a value measured by gel permeation chromatography (GPC).As measurement conditions, the following conditions can be used. Theweight-average molecular weight shown in examples which will bedescribed later is a value obtained by performing polystyrene conversionof a value measured under the following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In one aspect, as the binding agent, a binding agent including an acidicgroup can be used. The acidic group of the invention and thespecification is used as a meaning including a state of a group capableof emitting H⁺ in water or a solvent including water (aqueous solvent)to dissociate anions and salt thereof. Specific examples of the acidicgroup include a sulfonic acid group, a sulfuric acid group, a carboxygroup, a phosphoric acid group, and salt thereof. For example, salt ofsulfonic acid group (—SO₃H) is represented by —SO₃M, and M represents agroup representing an atom (for example, alkali metal atom or the like)which may be cations in water or in an aqueous solvent. The same appliesto aspects of salt of various groups described above. As an example ofthe binding agent including the acidic group, a resin including at leastone kind of acidic group selected from the group consisting of asulfonic acid group and salt thereof (for example, a polyurethane resinor a vinyl chloride resin) can be used. However, the resin included inthe magnetic layer is not limited to these resins. In addition, in thebinding agent including the acidic group, a content of the acidic groupcan be, for example, 20 to 500 eq/ton. eq indicates equivalent and SIunit is a unit not convertible. The content of various functional groupssuch as the acidic group included in the resin can be obtained by awell-known method in accordance with the kind of the functional group.As the binding agent having a great content of the acidic group is used,the value of ΔN tends to increase. The amount of the binding agent usedin a magnetic layer forming composition can be, for example, 1.0 to 30.0parts by mass, and preferably 1.0 to 20.0 parts by mass with respect to100.0 parts by mass of the ferromagnetic powder. As the amount of thebinding agent used with respect to the ferromagnetic powder increases,the value of ΔN tends to increase.

In addition, a curing agent can also be used together with the resinwhich can be used as the binding agent. As the curing agent, in oneaspect, a thermosetting compound which is a compound in which a curingreaction (crosslinking reaction) proceeds due to heating can be used,and in another aspect, a photocurable compound in which a curingreaction (crosslinking reaction) proceeds due to light irradiation canbe used. At least a part of the curing agent is included in the magneticlayer in a state of being reacted (crosslinked) with other componentssuch as the binding agent, by proceeding the curing reaction in themagnetic layer forming step. This point is the same as regarding a layerformed by using a composition, in a case where the composition used forforming the other layer includes the curing agent. The preferred curingagent is a thermosetting compound, polyisocyanate is suitable. Fordetails of the polyisocyanate, descriptions disclosed in paragraphs 0124and 0125 of JP2011-216149A can be referred to, for example. The amountof the curing agent can be, for example, 0 to 80.0 parts by mass withrespect to 100.0 parts by mass of the binding agent in the magneticlayer forming composition, and is preferably 50.0 to 80.0 parts by mass,from a viewpoint of improvement of hardness of the magnetic layer.

Additives

The magnetic layer includes ferromagnetic powder and a binding agent,and may include one or more kinds of additives, if necessary. As theadditives, the curing agent described above is used as an example. Inaddition, examples of the additive included in the magnetic layerinclude non-magnetic powder (for example, inorganic powder or carbonblack), a lubricant, a dispersing agent, a dispersing assistant, anantibacterial agent, an antistatic agent, and an antioxidant. As thenon-magnetic powder, non-magnetic powder which can function as anabrasive, non-magnetic powder (for example, non-magnetic colloidparticles) which can function as a projection formation agent whichforms projections suitably protruded from the surface of the magneticlayer, and the like can be used. An average particle size of colloidalsilica (silica colloid particles) shown in the examples which will bedescribed later is a value obtained by a method disclosed in ameasurement method of an average particle diameter in a paragraph 0015of JP2011-048878A. As the additives, a commercially available productcan be suitably selected according to the desired properties ormanufactured by a well-known method, and can be used with any amount. Asan example of the additive which can be used in the magnetic layerincluding the abrasive, a dispersing agent disclosed in paragraphs 0012to 0022 of JP2013-131285A can be used as a dispersing agent forimproving dispersibility of the abrasive. For example, for thelubricant, a description disclosed in paragraphs 0030 to 0033, 0035, and0036 of JP2016-126817A can be referred to. The non-magnetic layer mayinclude the lubricant. For the lubricant which may be included in thenon-magnetic layer, a description disclosed in paragraphs 0030, 0031,0034, 0035, and 0036 of JP2016-126817A can be referred to. For thedispersing agent, a description disclosed in paragraphs 0061 and 0071 ofJP2012-133837A can be referred to. The dispersing agent may be includedin the non-magnetic layer. For the dispersing agent which may beincluded in the non-magnetic layer, a description disclosed in aparagraph 0061 of JP2012-133837A can be referred to.

The magnetic layer described above can be provided on the surface of thenon-magnetic support directly or indirectly through the non-magneticlayer.

Non-Magnetic Layer

Next, the non-magnetic layer will be described.

The magnetic tape may include a magnetic layer directly on the surfaceof the non-magnetic support or may include a non-magnetic layerincluding the non-magnetic powder and the binding agent between thenon-magnetic support and the magnetic layer. The non-magnetic powderincluded in the non-magnetic layer may be inorganic powder or organicpowder. In addition, carbon black and the like can be used. Examples ofthe inorganic powder include powder of metal, metal oxide, metalcarbonate, metal sulfate, metal nitride, metal carbide, and metalsulfide. The non-magnetic powder can be purchased as a commerciallyavailable product or can be manufactured by a well-known method. Fordetails thereof, descriptions disclosed in paragraphs 0036 to 0039 ofJP2010-024113A can be referred to. The content (filling percentage) ofthe non-magnetic powder of the non-magnetic layer is preferably 50% to90% by mass and more preferably 60% to 90% by mass.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

The non-magnetic layer of the invention and the specification alsoincludes a substantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100Oe), or a layer having a residual magnetic flux density equal to orsmaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have aresidual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support (hereinafter, also simply referred to asa “support”) will be described.

As the non-magnetic support, well-known components such as polyethyleneterephthalate, polyethylene naphthalate, polyamide, polyamide imide,aromatic polyamide subjected to biaxial stretching are used. Amongthese, polyethylene terephthalate, polyethylene naphthalate, andpolyamide are preferable. Corona discharge, plasma treatment,easy-bonding treatment, or heat treatment may be performed with respectto these supports in advance.

Back Coating Layer

The magnetic tape can also include a back coating layer including anon-magnetic powder and a binding agent on a surface of the non-magneticsupport opposite to the surface provided with the magnetic layer. Theback coating layer preferably includes any one or both of carbon blackand inorganic powder. In regards to the binding agent included in theback coating layer and various additives which can be randomly includedtherein, a well-known technology regarding the back coating layer can beapplied, and a well-known technology regarding the list of the magneticlayer and/or the non-magnetic layer can also be applied. For example,for the back coating layer, descriptions disclosed in paragraphs 0018 to0020 of JP2006-331625A and page 4, line 65, to page 5, line 38, of U.S.Pat. No. 7,029,774B can be referred to.

Various Thicknesses

The thicknesses of the non-magnetic support and each layer of themagnetic tape will be described below.

The thickness of the non-magnetic support is, for example, 3.0 to 80.0μm, preferably 3.0 to 50.0 μm, and more preferably 3.0 to 10.0 μm.

A thickness of the magnetic layer can be optimized according to asaturation magnetization of a magnetic head used, a head gap length, arecording signal band, and the like. The thickness of the magnetic layeris normally 10 nm to 100 nm, and is preferably 20 to 90 nm and morepreferably 30 to 70 nm, from a viewpoint of realization of high-densityrecording. The magnetic layer may be at least one layer, or the magneticlayer can be separated to two or more layers having magnetic properties,and a configuration regarding a well-known multilayered magnetic layercan be applied. A thickness of the magnetic layer which is separatedinto two or more layers is a total thickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.1 to 1.5 μm andpreferably 0.1 to 1.0 μm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.9 μm and even more preferably 0.1 to 0.7 μm.

The thicknesses of various layers and the non-magnetic support areobtained by exposing a cross section of the magnetic tape in a thicknessdirection by a well-known method of ion beams or microtome, andobserving the exposed cross section with a scanning transmissionelectron microscope (STEM). For the specific examples of the measurementmethod of the thickness, a description disclosed regarding themeasurement method of the thickness in examples which will be describedlater can be referred to.

Manufacturing Step

Preparation of Each Layer Forming Composition

Steps of preparing the composition for forming the magnetic layer, thenon-magnetic layer, or the back coating layer generally include at leasta kneading step, a dispersing step, and a mixing step provided beforeand after these steps, if necessary. Each step may be divided into twoor more stages. The components used in the preparation of each layerforming composition may be added at an initial stage or in a middlestage of each step. As the solvent, one kind or two or more kinds ofvarious solvents generally used for manufacturing a coating typemagnetic recording medium can be used. For the solvent, a descriptiondisclosed in a paragraph 0153 of JP2011-216149A can be referred to, forexample. In addition, each component may be separately added in two ormore steps. For example, the binding agent may be separately added inthe kneading step, the dispersing step, and a mixing step for adjustinga viscosity after the dispersion. In order to manufacture the magnetictape, a well-known manufacturing technology of the related art can beused in various steps. In the kneading step, an open kneader, acontinuous kneader, a pressure kneader, or a kneader having a strongkneading force such as an extruder is preferably used. For details ofthe kneading processes, descriptions disclosed in JP1989-106338A(JP-H01-106338A) and JP1989-079274A (JP-H01-079274A) can be referred to.As a disperser, a well-known disperser can be used. In addition, theferromagnetic powder and the abrasive can also be dispersed separately.The separate dispersion is specifically a method of preparing a magneticlayer forming composition through a step of mixing an abrasive solutionincluding an abrasive and a solvent (here, ferromagnetic powder is notsubstantially included) with a magnetic liquid including theferromagnetic powder, a solvent, and a binding agent. The expression“ferromagnetic powder is not substantially included” means that theferromagnetic powder is not added as a constituent component of theabrasive solution, and a small amount of the ferromagnetic powder mixedas impurities without any intention is allowed. Regarding ΔN, as aperiod of the dispersion time of the magnetic liquid increases, thevalue of ΔN tends to increase. This is thought that, as a period of thedispersion time of the magnetic liquid increases, the dispersibility ofthe ferromagnetic powder in the coating layer of the magnetic layerforming composition increases, and the uniformity of the alignment stateof the ferromagnetic particles configuring the ferromagnetic powder bythe alignment process tends to easily increase. In addition, as theperiod of the dispersion time in a case of mixing and dispersing variouscomponents of the non-magnetic layer forming composition increases, thevalue of ΔN tends to increase. The dispersion time of the magneticliquid and the dispersion time of the non-magnetic layer formingcomposition may be set so that ΔN of 0.25 to 0.40 can be realized.

In any stage of preparing each layer forming composition, the filteringmay be performed by a well-known method. The filtering can be performedby using a filter, for example. As the filter used in the filtering, afilter having a hole diameter of 0.01 to 3 μm (for example, filter madeof glass fiber or filter made of polypropylene) can be used, forexample.

Coating Step

The non-magnetic layer and the magnetic layer can be formed byperforming multilayer coating with the non-magnetic layer formingcomposition and the magnetic layer forming composition in order or atthe same time. The back coating layer can be formed by applying the backcoating layer forming composition onto the surface of the non-magneticsupport opposite to the surface provided with the non-magnetic layer andthe magnetic layer (or non-magnetic layer and/or the magnetic layer isto be provided). In addition, the coating step for forming each layercan be also performed by being divided into two or more steps. Forexample, in one aspect, the magnetic layer forming composition can beapplied in two or more steps. In this case, a drying process may beperformed or may not be performed during the coating steps of twostages. In addition, the alignment process may be performed or may notbe performed during the coating steps of two stages. For details of thecoating for forming each layer, a description disclosed in a paragraph0066 of JP2010-231843A can be referred to. In addition, for the dryingstep after applying the each layer forming composition, a well-knowntechnology can be applied. Regarding the magnetic layer formingcomposition, as a drying temperature of a coating layer which is formedby applying the magnetic layer forming composition (hereinafter, alsoreferred to as a “coating layer of the magnetic layer formingcomposition” or simply a “coating layer”) decreases, the value of ΔNtends to increase. The drying temperature can be an atmospheretemperature for performing the drying step, for example, and may be setso that ΔN of 0.25 to 0.40 can be realized.

Other Steps

For various other steps for manufacturing the magnetic tape, awell-known technology can be applied. For details of the various steps,descriptions disclosed in paragraphs 0067 to 0070 of JP2010-231843A canbe referred to, for example.

For example, it is preferable to perform the alignment process withrespect to the coating layer of the magnetic layer forming compositionwhile the coating layer is wet. From a viewpoint of ease of exhibitingof ΔN of 0.25 to 0.40, the alignment process is preferably performed bydisposing a magnet so that a magnetic field is vertically applied to thesurface of the coating layer of the magnetic layer forming composition(that is, homeotropic alignment process). The strength of the magneticfield during the alignment process may be set so that ΔN of 0.25 to 0.40can be realized. In addition, in a case of performing the coating stepof the magnetic layer forming composition by the coating steps of two ormore stages, it is preferable to perform the alignment process at leastafter the final coating step, and it is more preferable to perform thehomeotropic alignment process. For example, in a case of forming themagnetic layer by the coating steps of two stages, the drying step isperformed without performing the alignment process after the firstcoating step, and then, the alignment process can be performed withrespect to the formed coating layer in the second coating step.

In addition, it is preferable to perform the calendar process in anystage after drying the coating layer of the magnetic layer formingcomposition. For the conditions of the calendar process, a descriptiondisclosed in a paragraph 0026 of JP2010-231843A can be referred to. Asthe calendar temperature (surface temperature of the calendar roll)increases, the value of ΔN tends to increase. The calendar temperaturemay be set so that ΔN of 0.25 to 0.40 can be realized.

One Aspect of Preferred Manufacturing Method

In a preferred aspect, a magnetic layer can be formed through a magneticlayer forming step including a coating step of applying a magnetic layerforming composition including ferromagnetic powder, a binding agent, acuring agent, and a solvent onto a non-magnetic support directly or witha non-magnetic layer interposed therebetween, to form a coating layer, aheating and drying step of drying the coating layer by a heatingprocess, and a curing step of performing a curing process with respectto the coating layer. The magnetic layer forming step preferablyincludes a cooling step of cooling the coating layer between the coatingstep and the heating and drying step, and more preferably includes aburnishing treatment step of performing a burnishing treatment withrespect to the surface of the coating layer between the heating anddrying step and the curing step.

It is thought that it is preferable that the cooling step and theburnishing treatment step in the magnetic layer forming step, in orderto set the logarithmic decrement to be equal to or smaller than 0.050.More specific description is as follows.

It is surmised that performing the cooling step of cooling the coatinglayer between the coating step and the heating and drying stepcontributes to causing pressure sensitive adhesive component describedabove is localized in the surface and/or a surface layer part in thevicinity of the surface of the coating layer. It is thought that this isbecause the pressure sensitive adhesive component at the time of solventvolatilization in the heating and drying step is easily moved to thesurface and/or the surface layer part of the coating layer, by coolingthe coating layer of the magnetic layer forming composition before theheating and drying step. However, the reason thereof is not clear. It isthought that the pressure sensitive adhesive component can be removed byperforming the burnishing treatment with respect to the surface of thecoating layer in which the pressure sensitive adhesive component islocalized on the surface and/or surface layer part. It is surmised thatperforming the curing step after removing the pressure sensitiveadhesive component contributes to setting the logarithmic decrement tobe equal to or smaller than 0.050. However, this is merely a surmise,and the invention is not limited thereto.

As described above, multilayer coating of the magnetic layer formingcomposition can be performed with the non-magnetic layer formingcomposition in order or at the same time. In a preferred aspect, themagnetic tape can be manufactured by successive multilayer coating. Amanufacturing step including the successive multilayer coating can bepreferably performed as follows. The non-magnetic layer is formedthrough a coating step of applying a non-magnetic layer formingcomposition onto a non-magnetic support to form a coating layer, and aheating and drying step of drying the formed coating layer by a heatingprocess. In addition, the magnetic layer is formed through a coatingstep of applying a magnetic layer forming composition onto the formednon-magnetic layer to form a coating layer, and a heating and dryingstep of drying the formed coating layer by a heating process.

Hereinafter, a specific aspect of the manufacturing method will bedescribed with reference to FIG. 4 . However, the invention is notlimited to the following specific aspect. Hereinafter, the coating layerof the magnetic layer forming composition before performing the curingprocess may be referred to as the magnetic layer. The same applies toother layers.

FIG. 4 is a schematic step view showing a specific aspect of a step ofmanufacturing the magnetic tape including a non-magnetic layer and amagnetic layer in this order on one surface of a non-magnetic supportand including a back coating layer on the other surface thereof. In theaspect shown in FIG. 4 , an operation of sending a non-magnetic support(elongated film) from a sending part and winding the non-magneticsupport around a winding part is continuously performed, and variousprocesses of coating, drying, and alignment are performed in each partor each zone shown in FIG. 4 , and thus, it is possible to sequentiallyform a non-magnetic layer and a magnetic layer on one surface of therunning non-magnetic support by multilayer coating and to form a backcoating layer on the other surface thereof. Such a manufacturing methodcan be set to be identical to the manufacturing method normallyperformed for manufacturing a coating type magnetic tape, except forincluding a cooling zone in the magnetic layer forming step andincluding the burnishing treatment step before the curing process.

The non-magnetic layer forming composition is applied onto thenon-magnetic support sent from the sending part in a first coating part(coating step of non-magnetic layer forming composition).

After the coating step, in a first heating process zone, the coatinglayer of the non-magnetic layer forming composition formed in thecoating step is heated after to dry the coating layer (heating anddrying step). The heating and drying step can be performed by causingthe non-magnetic support including the coating layer of the non-magneticlayer forming composition to pass through the heated atmosphere. Anatmosphere temperature of the heated atmosphere here can be, forexample, approximately 40° C. to 140° C. Here, the atmospheretemperature may be a temperature at which the solvent is volatilized andthe coating layer is dried, and the atmosphere temperature is notlimited to the range described above. In addition, the heated air mayblow to the surface of the coating layer. The points described above arealso applied to a heating and drying step of a second heating processzone and a heating and drying step of a third heating process zone whichwill be described later, in the same manner.

Next, in a second coating part, the magnetic layer forming compositionis applied onto the non-magnetic layer formed by performing the heatingand drying step in the first heating process zone (coating step ofmagnetic layer forming composition).

After the coating step, a coating layer of the magnetic layer formingcomposition formed in the coating step is cooled in a cooling zone(cooling step). For example, it is possible to perform the cooling stepby allowing the non-magnetic support on which the coating layer isformed on the non-magnetic layer to pass through a cooling atmosphere.An atmosphere temperature of the cooling atmosphere is preferably −10°C. to 0° C. and more preferably −5° C. to 0° C. The time for performingthe cooling step (for example, time while a random part of the coatinglayer is delivered to and sent from the cooling zone (hereinafter, alsoreferred to as a “staying time”)) is not particularly limited. In a casewhere the staying time is long, the logarithmic decrement tends to bedecreased. Thus, the staying time is preferably adjusted by performingpreliminary experiment if necessary, so that the logarithmic decrementequal to or smaller than 0.050 is realized. In the cooling step, cooledair may blow to the surface of the coating layer.

After that, in the aspect of performing the alignment process, while thecoating layer of the magnetic layer forming composition is wet, analignment process of the ferromagnetic powder in the coating layer isperformed in an alignment zone. Regarding the alignment process, theabove description can also be referred to.

The coating layer after the alignment process is subjected to theheating and drying step in the second heating process zone.

Next, in the third coating part, a back coating layer formingcomposition is applied to a surface of the non-magnetic support on aside opposite to the surface where the non-magnetic layer and themagnetic layer are formed, to form a coating layer (coating step of backcoating layer forming composition). After that, the coating layer isheated and dried in the third heating process zone.

By doing so, it is possible to obtain the magnetic tape including themagnetic layer on the non-magnetic layer on one surface of thenon-magnetic support and the back coating layer on the other surfacethereof. The magnetic tape obtained here becomes a magnetic tape productafter performing various processes which will be described later.

The obtained magnetic tape is wound around the winding part, and cut(slit) to have a size of a magnetic tape product. The slitting isperformed by using a well-known cutter.

In the slit magnetic tape, the burnishing treatment is performed withrespect to the surface of the magnetic layer, before performing thecuring process (heating and light irradiation) in accordance with thetypes of the curing agent included in the magnetic layer (burnishingtreatment step between heating and drying step and curing step). Theinventors have surmised that removing the pressure sensitive adhesivecomponent transitioned to the surface and/or the surface layer part ofthe magnetic layer cooled in the cooling zone by the burnishingtreatment contributes to setting the logarithmic decrement to be equalto or smaller than 0.050. However, this is merely a surmise, and theinvention is not limited thereto.

The burnishing treatment is treatment of rubbing a surface of atreatment target with a member (for example, a polishing tape, or agrinding tool such as a grinding blade or a grinding wheel), and can beperformed in the same manner as the well-known burnishing treatment formanufacturing a coating type magnetic recording medium. However, in therelated art, the burnishing treatment was not performed in a stagebefore the curing step, after performing the cooling step and theheating and drying step. With respect to this, the logarithmic decrementcan be equal to or smaller than 0.050 by performing the burnishingtreatment in the stage described above.

The burnishing treatment can be preferably performed by performing oneor both of rubbing of the surface of the layer of the treatment targetby a polishing tape (polishing) and rubbing of the surface of the layerof the treatment target by a grinding tool (grinding). As the polishingtape, a commercially available product may be used and a polishing tapemanufactured by a well-known method may be used. As the grinding tool, awell-known blade such as a fixed blade, a diamond wheel, or a rotaryblade, or a grinding wheel can be used. In addition, a wiping treatmentof wiping the surface of the layer rubbed by the polishing tape and/orthe grinding tool with a wiping material. For details of preferredpolishing tape, grinding tool, burnishing treatment, and wipingtreatment, descriptions disclosed in paragraphs 0034 to 0048, FIG. 1 andexamples of JP1994-052544A (JP-H06-052544A) can be referred to. As theburnishing treatment is reinforced, the value of the logarithmicdecrement tends to be decreased. The burnishing treatment can bereinforced as an abrasive having high hardness is used as the abrasiveincluded in the polishing tape, and can be reinforced, as the amount ofthe abrasive in the polishing tape is increased. In addition, theburnishing treatment can be reinforced as a grinding tool having highhardness is used as the grinding tool. In regards to the burnishingtreatment conditions, the burnishing treatment can be reinforced as asliding speed between the surface of the layer of the treatment targetand a member (for example, a polishing tape or a grinding tool) isincreased. The sliding speed can be increased by increasing one or bothof a speed at which the member is moved, and a speed at which themagnetic tape of the treatment target is moved.

After the burnishing treatment (burnishing treatment step), the curingprocess is performed with respect to the magnetic layer. In the aspectshown in FIG. 4 , the magnetic layer is subjected to the surfacesmoothing treatment, after the burnishing treatment and before thecuring process. The surface smoothing treatment is preferably performedby a calendar process.

After that, the curing process according to the type of the curing agentincluded in this layer is performed with respect to the magnetic layer(curing step). The curing process can be performed by the processaccording to the type of the curing agent included in the coating layer,such as a heating process or light irradiation. The curing processconditions are not particularly limited, and the curing processconditions may be suitably set in accordance with the list of themagnetic layer forming composition, the type of the curing agent, andthe thickness of the magnetic layer. For example, in a case where themagnetic layer is formed by using the magnetic layer forming compositionincluding polyisocyanate as the curing agent, the curing process ispreferably the heating process. In a case where the curing agent isincluded in a layer other than the magnetic layer, a curing reaction ofthe layer can also be promoted by the curing process here.Alternatively, the curing step may be separately provided. After thecuring step, the burnishing treatment may be further performed.

As described above, it is possible to obtain the magnetic tape accordingto one aspect of the invention. However, the manufacturing methoddescribed above is merely an example, each value can be controlled to bein respective ranges described above by any means capable of adjustingΔN and the logarithmic decrement, and such an aspect is also included inthe invention. The magnetic tape is normally accommodated in a magnetictape cartridge and the magnetic tape cartridge is mounted on a magneticrecording and reproducing device. In a case of reproducing informationrecorded on the magnetic tape in the magnetic recording and reproducingdevice in the low temperature and high humidity environment, it ispossible to decrease the generation frequency of the missing pulse, in acase of using the magnetic tape according to one aspect of theinvention.

In the magnetic tape thus prepared, a servo pattern may be formed by aknown method, in order to allow control of tracking of a magnetic headand control of the running speed of the magnetic recording medium to beperformed in the magnetic recording and reproducing device. The“formation of a servo pattern” can also be referred to as “recording ofa servo signal”. Formation of the servo pattern in a magnetic tape willbe described below, as an example.

The servo pattern is generally recorded along the longitudinal directionof the magnetic tape. Examples of control (servo control) systemsutilizing servo signals include timing-based servo (TBS), amplitudeservo, and frequency servo.

As shown in European Computer Manufacturers Association (ECMA)-319, atiming-based servo technique has been employed in a magnetic tape(generally referred to as “LTO tape”) in accordance with LinearTape-Open (LTO) specifications. In this timing-based servo technique,the servo patterns are configured of consecutive alignment of aplurality of pairs of magnetic stripes (also referred to as “servostripes”), in each pair of which magnetic stripes are not parallel witheach other, in the longitudinal direction of the magnetic tape. Thereason why the servo signal is configured of pairs of magnetic stripes,in each pair of which magnetic stripes are not parallel with each other,is to teach a passing position to a servo signal reading element passingover the servo pattern. Specifically, the pairs of magnetic stripes areformed so that the intervals consecutively change along the widthdirection of the magnetic tape, and relative positions of the servopattern and the servo signal reading element can be determined byreading the intervals with the servo signal reading element. Theinformation on this relative positions enable the data track to betracked. Thus, a plurality of servo tracks are generally set over theservo signal along the width direction of the magnetic tape.

The servo band is configured of servo signals continuously aligned inthe longitudinal direction of the magnetic tape. A plurality of theservo bands are generally provided in the magnetic tape. For example, inan LTO tape, the number of servo bands is five. A region sandwichedbetween the adjacent two servo bands is referred to as a data band. Thedata band is configured of a plurality of data tracks, and data trackscorresponds to respective servo tracks.

In one aspect, information on the number of servo bands (also referredto as information on a “servo band identification (ID)” or a “uniquedata band identification method (UDIM)”) is embedded in each servo bandas shown in Japanese Patent Application Publication No. 2004-318983.This servo band ID is recorded shiftedly such that the position of aspecific pair of servo stripes, among a plurality of servo stripespresent in a servo band, should shift in the longitudinal direction ofthe magnetic tape. Specifically, the degree of shifting the specificpair of servo stripes among the plurality of pairs of servo stripes ischanged by each servo band. Accordingly, the recorded servo band ID isunique by each servo band, and the servo band is uniquely specified byreading one servo band with the servo signal reading element.

As another method for uniquely specifying a servo band, a method using astaggered technique as shown in ECMA-319 can be applied. In thisstaggered technique, a group of a plurality of pairs of magnetic stripes(servo stripes), in each pair of which magnetic stripes are not parallelwith each other and which are placed consecutively in the longitudinaldirection of the magnetic tape, are shiftedly recorded by each servoband in the longitudinal direction of the magnetic tape. A combinationof ways of shifting for each adjacent servo bands is unique in theentire magnetic tape. Accordingly, when a servo pattern is read with twoservo signal reading elements, the servo band can be uniquely specified.

Information indicating a position in the longitudinal direction of themagnetic tape (also referred to as “longitudinal position (LPOS)information”) is also generally embedded in each servo band as shown inECMA-319. This LPOS information is also recorded by shifting theposition of the pair of servo stripes in the longitudinal direction ofthe magnetic tape. Unlike the UDIM information, the same signal isrecorded in each servo band in the case of LPOS information.

Other information different from UDIM information and LPOS informationas mentioned above can also be embedded in the servo band. In this case,the information to be embedded may be different by each servo band likethe UDIM information or may be the same by each servo band like the LPOSinformation.

As a method for embedding information in a servo band, a method otherthan the above-described method may also be employed. For example, amonga group of pairs of servo stripes, a predetermined pair of servo stripesis thinned out to record a predetermined code.

A head for forming a servo pattern is referred to as a servo write head.The servo write head has the same number of pairs of gaps correspondingto the respective pairs of magnetic stripes as the number of servobands. Generally, a core and a coil are connected to each pair of gaps,and a magnetic field generated in the core by suppling a current pulseto the coil can generate a leakage magnetic field to the pair of gaps.When a servo pattern is formed, a magnetic pattern corresponding to apair of gaps can be transferred to the magnetic tape by inputting acurrent pulse while causing a magnetic tape to run over the servo writehead, to form a servo pattern. Thus, the servo pattern can be formed.The width of each gap can be set as appropriate according to the densityof the servo pattern to be formed. The width of each gap can be set to,for example, 1 μm or less, 1 to 10 μm, or 10 μm or larger.

Before forming a servo pattern on the magnetic tape, the magnetic tapeis generally subjected to a demagnetization (erasing) treatment. Thiserasing treatment may be performed by adding a uniform magnetic field tothe magnetic tape using a direct current magnet or an alternate currentmagnet. The erasing treatment includes direct current (DC) erasing andan alternating current (AC) erasing. The AC erasing is performed bygradually reducing the intensity of the magnetic field while invertingthe direction of the magnetic field applied to the magnetic tape. Incontrast, the DC erasing is performed by adding a one-direction magneticfield to the magnetic tape. The DC erasing further includes two methods.The first method is horizontal DC erasing of applying a one-directionmagnetic field along the longitudinal direction of the magnetic field.The second method is a vertical DC erasing of applying a one-directionmagnetic field along the thickness direction of the magnetic tape. Theerasing treatment may be applied to the entire magnetic tape of themagnetic tape, or may be applied to each servo band of the magnetictape.

The direction of the magnetic field of the servo pattern to be formed isdetermined according to the direction of the erasing. For example, whenthe magnetic tape has been subjected to the horizontal DC erasing, theservo pattern is formed so that the direction of the magnetic fieldbecomes reverse to the direction of the erasing. Accordingly, the outputof the servo signal, which can be yielded by reading the servo pattern,can be increased. As shown in Japanese Patent Application PublicationNo. 2012-53940, when a magnetic pattern is transferred to the magnetictape which has been subjected to the vertical DC erasing using the gaps,the servo signal, which has been yielded by reading the servo patternthus formed, has a unipolar pulse shape. In contrast, when a magneticpattern is transferred to the magnetic tape which has been subjected tothe parallel DC erasing, the servo signal, which has been yielded byreading the servo pattern thus formed, has a bipolar pulse shape.

Magnetic Recording and Reproducing Device

Another aspect of the invention relates to a magnetic recording andreproducing device including the magnetic tape and a magnetic head.

In the invention and the specification, the “magnetic recording andreproducing device” means a device capable of performing at least one ofthe recording of information on the magnetic tape or the reproducing ofinformation recorded on the magnetic tape. Such a device is generallycalled a drive. The magnetic head included in the magnetic recording andreproducing device can be a recording head capable of performing therecording of information on the magnetic tape, and can also be areproducing head capable of performing the reproducing of informationrecorded on the magnetic tape. In addition, in one aspect, the magneticrecording and reproducing device can include both of a recording headand a reproducing head as separate magnetic heads. In another aspect,the magnetic head included in the magnetic recording and reproducingdevice can also have a configuration of comprising both of a recordingelement and a reproducing element in one magnetic head. As thereproducing head, a magnetic head (MR head) including a magnetoresistive(MR) element capable of reading information recorded on the magnetictape with excellent sensitivity as the reproducing element ispreferable. As the MR head, various well-known MR heads can be used. Inaddition, the magnetic head which performs the recording of informationand/or the reproducing of information may include a servo patternreading element. Alternatively, as a head other than the magnetic headwhich performs the recording of information and/or the reproducing ofinformation, a magnetic head (servo head) comprising a servo patternreading element may be included in the magnetic recording andreproducing device.

In the magnetic recording and reproducing device, the recording ofinformation on the magnetic tape and the reproducing of informationrecorded on the magnetic tape can be performed by bringing the surfaceof the magnetic layer of the magnetic tape into contact with themagnetic head and sliding. The magnetic recording and reproducing devicemay include the magnetic tape according to one aspect of the invention,and well-known technologies can be applied for the other configurations.

The magnetic recording and reproducing device includes the magnetic tapeaccording to one aspect of the invention. Therefore, it is possible todecrease the generation frequency of the missing pulse in a case ofreproducing information recorded on the magnetic tape in the lowtemperature and high humidity environment. In addition, even in a casewhere the surface of the magnetic layer and the head slide on each otherfor recording information on the magnetic tape in the low temperatureand high humidity environment, it is possible to prevent the unstablecontact state of the surface of the magnetic layer and the head due tothe head attached material caused by the chipping of the surface of themagnetic layer.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description are based on mass.

Example 1

Preparation of Abrasive Solution

3.0 parts of 2,3-dihydroxynaphthalene (manufactured by Tokyo ChemicalIndustry Co., Ltd.), 31.3 parts of a 32% solution (solvent is a mixedsolvent of methyl ethyl ketone and toluene) of a SO₃Na group-containingpolyester polyurethane resin (UR-4800 (SO₃Na group: 0.08 meq/g)manufactured by Toyobo Co., Ltd.), and 570.0 parts of a mixed solvent ofmethyl ethyl ketone and cyclohexanone (mass ratio of 1:1) as a solventwere mixed with 100.0 parts of alumina powder (HIT-80 manufactured bySumitomo Chemical Co., Ltd.) having a gelatinization ratio of 65% and aBrunauer-Emmett-Teller (BET) specific surface area of 20 m²/g, anddispersed in the presence of zirconia beads by a paint shaker for 5hours. After the dispersion, the dispersion liquid and the beads wereseparated by a mesh and an alumina dispersion was obtained.

Preparation of Magnetic Layer Forming Composition

-   -   Magnetic Liquid    -   Plate-shaped ferromagnetic hexagonal barium ferrite powder:        100.0 parts        -   (Activation volume: 1600 nm³, average plate ratio: 3.5)    -   SO₃Na group-containing polyurethane resin: see Table 1        -   (Weight-average molecular weight: 70,000, SO₃Na group: see            Table 1)    -   Cyclohexanone: 150.0 parts    -   Methyl ethyl ketone: 150.0 parts    -   Abrasive Solution    -   Alumina dispersion prepared as described above: 6.0 parts    -   Silica Sol (projection forming agent liquid)    -   Colloidal silica (Average particle size: 100 nm): 2.0 parts    -   Methyl ethyl ketone: 1.4 parts    -   Other Components    -   Stearic acid: 2.0 parts    -   Butyl stearate: 2.0 parts    -   Polyisocyanate (CORONATE (registered trademark) manufactured by        Tosoh Corporation): 2.5 parts    -   Finishing Additive Solvent    -   Cyclohexanone: 200.0 parts    -   Methyl ethyl ketone: 200.0 parts

Preparation Method

The magnetic liquid was prepared by beads-dispersing of variouscomponents of the magnetic liquid described above by using beads as thedispersion medium in a batch type vertical sand mill. The beaddispersion was performed using zirconia beads (bead diameter: seeTable 1) as the beads for the time shown in Table 1 (magnetic liquidbead dispersion time).

The magnetic liquid obtained as described above, the abrasive solution,silica sol, other components, and a finishing additive solvent weremixed with each other and beads-dispersed for 5 minutes, and thetreatment (ultrasonic dispersion) was performed with a batch typeultrasonic device (20 kHz, 300 W) for 0.5 minutes. After that, theobtained mixed solution was filtered by using a filter having a holediameter of 0.5 μm, and the magnetic layer forming composition wasprepared.

Preparation of Non-Magnetic Layer Forming Composition

Each component among various components of the non-magnetic layerforming composition shown below excluding stearic acid, cyclohexanone,and methyl ethyl ketone was beads-dispersed (dispersion medium: zirconiabeads (bead diameter: 0.1 mm), dispersion time: see Table 1) by using abatch type vertical sand mill to obtain a dispersion liquid. After that,the remaining components were added into the obtained dispersion liquidand stirred with a dissolver. Then, the obtained dispersion liquid wasfiltered with a filter (hole diameter: 0.5 μm) and a non-magnetic layerforming composition was prepared.

Non-magnetic inorganic powder: α-iron oxide: 100.0 parts

-   -   (Average particle size: 10 nm, BET specific surface area: 75        m²/g)

Carbon black: 25.0 parts

-   -   (Average particle size: 20 nm)

A SO₃Na group-containing polyurethane resin: 18.0 parts

-   -   (Weight-average molecular weight: 70,000, content of SO₃Na        group: 0.2 meq/g)

Stearic acid: 1.0 parts

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

Preparation of Back Coating Layer Forming Composition

Each component among various components of the back coating layerforming composition shown below excluding stearic acid, butyl stearate,polyisocyanate, and cyclohexanone was kneaded and diluted by an openkneader, and a mixed solution was obtained. After that, the obtainedmixed solution was subjected to a dispersion process of 12 passes, witha transverse beads mill and zirconia beads having a bead diameter of 1.0mm, by setting a bead filling percentage as 80 volume %, acircumferential speed of rotor distal end as 10 m/sec, and a retentiontime for 1 pass as 2 minutes. After that, the remaining components wereadded into the obtained dispersion liquid and stirred with a dissolver.Then, the obtained dispersion liquid was filtered with a filter (holediameter: 1.0 μm) and a back coating layer forming composition wasprepared.

Non-magnetic inorganic powder: α-iron oxide: 80.0 parts

-   -   Average particle size (average long axis length): 0.15 μm    -   Average acicular ratio: 7    -   BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

-   -   Average particle size: 20 nm

A vinyl chloride copolymer: 13.0 parts

A sulfonic acid salt group-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Methyl ethyl ketone: 155.0 parts

Stearic acid: 3.0 parts

Butyl stearate: 3.0 parts

Polyisocyanate: 5.0 parts

Cyclohexanone: 355.0 parts

Manufacturing of Magnetic Tape

A magnetic tape was manufactured by the specific aspect shown in FIG. 4. The magnetic tape was specifically manufactured as follows.

A support made of polyethylene naphthalate having a thickness of 5.0 μmwas sent from the sending part, and the non-magnetic layer formingcomposition was applied to one surface thereof so that the thicknessafter the drying becomes 0.7 μm in the first coating part, and dried inthe first heating process zone (atmosphere temperature of 100° C.) toform a non-magnetic layer.

After that, the magnetic layer forming composition was applied on thenon-magnetic layer so that the thickness after the drying becomes 50 nmin the second coating part to form the coating layer. The cooling stepwas performed by passing the formed coating layer through the coolingzone in which the atmosphere temperature is adjusted to 0° C. for thestaying time shown in Table 1 while the coating layer is wet. Then, ahomeotropic alignment process was performed by applying a magnetic fieldhaving a strength shown in Table 1 in a vertical direction with respectto the surface of the coating layer of the magnetic layer formingcomposition in the alignment zone, the coating layer was dried in thesecond heating process zone (atmosphere temperature: magnetic layerdrying temperature in Table 1) to form the magnetic layer.

After that, in the third coating part, the back coating layer formingcomposition was applied to the surface of the support made ofpolyethylene naphthalate on a side opposite to the surface where thenon-magnetic layer and the magnetic layer are formed, so that thethickness after the drying becomes 0.5 μm, to form a coating layer, andthe formed coating layer was dried in a third heating process zone(atmosphere temperature of 100° C.) to form the back coating layer.

The magnetic tape obtained as described above was slit to have a widthof ½ inches (0.0127 meters), and the burnishing treatment and the wipingtreatment were performed with respect to the surface of the magneticlayer. The burnishing treatment and the wiping treatment were performedby using a commercially available polishing tape (product name: MA22000manufactured by Fujifilm Corporation, abrasive: diamond/Cr₂O₃/red oxide)as the polishing tape, a commercially available sapphire blade(manufactured by Kyocera Corporation, a width of 5 mm, a length of 35mm, and a tip angle of 60 degrees) as the grinding blade, and acommercially available wiping material (product name: WRP736manufactured by Kuraray Co., Ltd.) as the wiping material, in atreatment device having a configuration disclosed in FIG. 1 ofJP1994-052544A (JP-H06-052544A). For the treatment conditions, thetreatment conditions disclosed in Example 12 of JP1994-052544A(JP-H06-052544A).

After the burnishing treatment and the wiping treatment, a calendarprocess (surface smoothing treatment) was performed with a calendar rollconfigured of only a metal roll, at a speed of 80 m/min, linear pressureof 300 kg/cm (294 kN/m), and a calendar temperature (surface temperatureof a calendar roll) shown in Table 1.

Then, a heating process (curing process) was performed in theenvironment of the atmosphere temperature of 70° C. for 36 hours, andthen, a servo pattern was formed on the magnetic layer by a commerciallyavailable servo writer.

By doing so, a magnetic tape of Example 1 was obtained.

Example 4 and Comparative Examples 1 to 6

A magnetic tape was manufactured in the same manner as in Example 1,except that various conditions shown in Table 1 were changed as shown inTable 1.

In Table 1, in the comparative examples in which “no alignment process”is shown in the column of “formation and alignment of magnetic layer”,the magnetic tape was manufactured without performing the alignmentprocess regarding the coating layer of the magnetic layer formingcomposition.

In Table 1, in the comparative examples in which “not performed” isshown in the column of the cooling zone staying time and the column ofthe burnishing treatment before the curing process, a magnetic tape wasmanufactured by a manufacturing step not including a cooling zone in themagnetic layer forming step and not performing the burnishing treatmentand the wiping treatment before the curing process.

Example 2

After forming the non-magnetic layer, the magnetic layer formingcomposition was applied on the surface of the non-magnetic layer so thatthe thickness after drying becomes 25 nm to form a first coating layer.The first coating layer was passed through the atmosphere at theatmosphere temperature shown in Table 1 (magnetic layer dryingtemperature) without application of a magnetic field to form a firstmagnetic layer (no alignment process).

After that, the magnetic layer forming composition was applied on thesurface of the first magnetic layer so that the thickness after dryingbecomes 25 nm to form a second coating layer. The cooling step wasperformed by causing the formed second coating layer to pass the coolingzone in which the atmosphere temperature was adjusted to 0° C. for thestaying time shown in Table 1, while the second coating layer was wet.After that, the homeotropic alignment process was performed by applyinga magnetic field having a strength shown in Table 1 in the verticaldirection with respect to the surface of the second coating layer in thealignment zone, and drying was performed in the second heating processzone (atmosphere temperature: magnetic layer drying temperature inTable 1) to form a second magnetic layer.

A magnetic tape was manufactured in the same manner as in Example 1,except that the multilayered magnetic layer was formed as describedabove.

Example 3

A magnetic tape was manufactured in the same manner as in Example 2,except that the cooling zone staying time was changed as shown in Table1.

Comparative Example 7

After forming the non-magnetic layer, the magnetic layer formingcomposition was applied on the surface of the non-magnetic layer so thatthe thickness after drying becomes 25 nm to form a first coating layer.The homeotropic alignment process and the drying process were performedby applying a magnetic field having a strength shown in Table 1 in thevertical direction with respect to the surface of the first coatinglayer using opposing magnet in the atmosphere at the atmospheretemperature shown in Table 1 (magnetic layer drying temperature) whilethe first coating layer was wet, and a first magnetic layer was formed.

After that, the magnetic layer forming composition was applied on thesurface of the first magnetic layer so that the thickness after dryingbecomes 25 nm to form a second coating layer. The second coating layerwas passed through the atmosphere at the atmosphere temperature shown inTable 1 (magnetic layer drying temperature) without application of amagnetic field to form a second magnetic layer (no alignment process).

A magnetic tape was manufactured in the same manner as in ComparativeExample 2, except that the multilayered magnetic layer was formed asdescribed above.

Comparative Example 8

After forming the non-magnetic layer, the magnetic layer formingcomposition was applied on the surface of the non-magnetic layer so thatthe thickness after drying becomes 25 nm to form a first coating layer.The homeotropic alignment process and the drying process were performedby applying a magnetic field having a strength shown in Table 1 in thevertical direction with respect to the surface of the first coatinglayer using opposing magnet in the atmosphere at the atmospheretemperature shown in Table 1 (magnetic layer drying temperature) whilethe first coating layer was wet, and a first magnetic layer was formed.

After that, the magnetic layer forming composition was applied on thesurface of the first magnetic layer so that the thickness after dryingbecomes 25 nm to form a second coating layer. The second coating layerwas passed through the atmosphere at the atmosphere temperature shown inTable 1 (magnetic layer drying temperature) without application of amagnetic field to form a second magnetic layer (no alignment process).

A magnetic tape was manufactured in the same manner as in ComparativeExample 6, except that the multilayered magnetic layer was formed asdescribed above, and the magnetic tape was manufactured by themanufacturing step not including the cooling zone in the magnetic layerforming step and not performing the burnishing treatment and the wipingtreatment before the curing process.

Comparative Example 9

After forming the non-magnetic layer, the magnetic layer formingcomposition was applied on the surface of the non-magnetic layer so thatthe thickness after drying becomes 25 nm to form a first coating layer.The first coating layer was passed through the atmosphere at theatmosphere temperature shown in Table 1 (magnetic layer dryingtemperature) without application of a magnetic field to form a firstmagnetic layer (no alignment process).

After that, the magnetic layer forming composition was applied on thesurface of the first magnetic layer so that the thickness after dryingbecomes 25 nm to form a second coating layer. The homeotropic alignmentprocess and the drying process were performed by applying a magneticfield having a strength shown in Table 1 in the vertical direction withrespect to the surface of the second coating layer using opposing magnetin the atmosphere at the atmosphere temperature shown in Table 1(magnetic layer drying temperature) while the second coating layer waswet, and a second magnetic layer was formed.

A magnetic tape was manufactured in the same manner as in ComparativeExample 3, except that the multilayered magnetic layer was formed asdescribed above.

Evaluation of Physical Properties of Magnetic Tape

(1) Measurement of Logarithmic Decrement of Magnetic Layer

The logarithmic decrement of the magnetic layer of the magnetic tape wasacquired by the method described above by using a rigid-body pendulumtype physical properties testing instrument RPT-3000W manufactured byA&D Company, Limited (pendulum: brass, substrate: glass substrate, arate of temperature increase of substrate: 5° C./min) as the measurementdevice. A measurement sample cut out from the magnetic tape was placedon a glass substrate having a size of approximately 3 cm× approximately5 cm, by being fixed at 4 portions with a fixing tape (Kapton tapemanufactured by Du Pont-Toray Co., Ltd.) as shown in FIG. 1 . Anadsorption time was set as 1 second, a measurement interval was set as 7to 10 seconds, a displacement-time curve was drawn regarding the 86-thmeasurement interval, and the logarithmic decrement was acquired byusing this curve. The measurement was performed in the environment ofrelative humidity of approximately 50%.

(2) Thicknesses of Non-Magnetic Support and Each Layer

The thicknesses of the magnetic layer, the non-magnetic layer, thenon-magnetic support, and the back coating layer of each manufacturedmagnetic tape were measured by the following method. As a result of themeasurement, in all of the magnetic tapes, the thickness of the magneticlayer was 50 nm, the thickness of the non-magnetic layer was 0.7 μm, thethickness of the non-magnetic support was 5.0 μm, and the thickness ofthe back coating layer was 0.5 μm.

The thicknesses of the magnetic layer, the non-magnetic layer, and thenon-magnetic support measured here were used for calculating thefollowing refractive index.

(i) Manufacturing of Cross Section Observation Sample

A cross section observation sample including all regions of the magnetictape from the magnetic layer side surface to the back coating layer sidesurface in the thickness direction was manufactured according to themethod disclosed in paragraphs 0193 and 0194 of JP2016-177851A.

(ii) Thickness Measurement

The manufactured sample was observed with the STEM and a STEM image wascaptured. This STEM image was a STEM-high-angle annular dark field(HAADF) image which is captured at an acceleration voltage of 300 kV anda magnification ratio of imaging of 450,000, and the imaging wasperformed so that entire region of the magnetic tape from the magneticlayer side surface to the back coating layer side surface in thethickness direction in one image. In the STEM image obtained asdescribed above, a linear line connecting both ends of a line segmentshowing the surface of the magnetic layer was determined as a referenceline showing the surface of the magnetic tape on the magnetic layerside. In a case where the STEM image was captured so that the magneticlayer side of the cross section observation sample was positioned on theupper side of the image and the back coating layer side was positionedon the lower side, for example, the linear line connecting both ends ofthe line segment described above is a linear line connecting anintersection between a left side of the image (shape is a rectangular orsquare shape) of the STEM image and the line segment, and anintersection between a right side of the STEM image and the line segmentto each other. In the same manner as described above, a reference lineshowing the interface between the magnetic layer and the non-magneticlayer, a reference line showing the interface between the non-magneticlayer and the non-magnetic support, a reference line showing theinterface between the non-magnetic support and the back coating layer,and a reference line showing the surface of the magnetic tape on theback coating layer side were determined.

The thickness of the magnetic layer was obtained as the shortestdistance from one position randomly selected on the reference lineshowing the surface of the magnetic tape on the magnetic layer side tothe reference line showing the interface between the magnetic layer andthe non-magnetic layer. In the same manner as described above, thethicknesses of the non-magnetic layer, the non-magnetic support, and theback coating layer were obtained.

(3) ΔN of Magnetic Layer

Hereinafter, M-2000U manufactured by J. A. Woollam Co., Inc. was used asthe ellipsometer. The creating and fitting of a double-layer model or asingle-layer model were performed with WVASE32 manufactured by J. A.Woollam Co., Inc. as the analysis software.

(i) Measurement Refractive Index of Non-Magnetic Support

A sample for measurement was cut out from each magnetic tape, the backcoating layer of the sample for measurement was wiped off and removedusing cloth permeated with methyl ethyl ketone to expose the surface ofthe non-magnetic support, and then, this surface is roughened with sandpaper so that reflected light of the exposed surface is not detected inthe measurement which will be performed after this with theellipsometer.

After that, by wiping off and removing the magnetic layer and thenon-magnetic layer of the sample for measurement using the clothpermeated with methyl ethyl ketone and bonding a surface of a siliconwafer and the roughened surface to each other using static electricity,the sample for measurement was disposed on the silicon wafer so that thesurface of the non-magnetic support exposed by removing the magneticlayer and the non-magnetic layer (hereinafter, referred to as the“surface of the non-magnetic support on the magnetic layer side”) facedupward.

The incidence ray was incident to the surface of the non-magneticsupport of the sample for measurement on the magnetic layer side on thesilicon wafer using the ellipsometer as described above, to measure Δand ψ. By using the obtained measurement values and the thickness of thenon-magnetic support obtained in the section (2), the refractive indexesof the non-magnetic support (the refractive index in a longitudinaldirection, the refractive index in a width direction, the refractiveindex in a thickness direction measured by incidence of incidence ray inthe longitudinal direction, and the refractive index in a thicknessdirection measured by incidence of incidence ray in the width direction)were obtained by the method described above.

(ii) Measurement of Refractive Index of Non-Magnetic Layer

A sample for measurement was cut out from each magnetic tape, the backcoating layer of the sample for measurement was wiped off and removedusing cloth permeated with methyl ethyl ketone to expose the surface ofthe non-magnetic support, and then, this surface is roughened with sandpaper so that reflected light of the exposed surface is not detected inthe measurement which will be performed after this with thespectroscopic ellipsometer.

After that, the surface of the magnetic layer of the sample formeasurement was wiped off using the cloth permeated with methyl ethylketone, the magnetic layer was removed to expose the surface of thenon-magnetic layer, and then, the sample for measurement was disposed onthe silicon wafer in the same manner as in the section (i).

The measurement regarding the surface of the non-magnetic layer of thesample for measurement on the silicon wafer was performed using theellipsometer, and the refractive indexes of the non-magnetic layer (therefractive index in a longitudinal direction, the refractive index in awidth direction, the refractive index in a thickness direction measuredby incidence of incidence ray in the longitudinal direction, and therefractive index in a thickness direction measured by incidence ofincidence ray in the width direction) were obtained by the methoddescribed above by spectral ellipsometry.

(iii) Measurement of Refractive Index of Magnetic Layer

A sample for measurement was cut out from each magnetic tape, the backcoating layer of the sample for measurement was wiped off and removedusing cloth permeated with methyl ethyl ketone to expose the surface ofthe non-magnetic support, and then, this surface is roughened with sandpaper so that reflected light of the exposed surface is not detected inthe measurement which will be performed after this with thespectroscopic ellipsometer.

After that, the sample for measurement was disposed on the sample formeasurement on the silicon wafer, in the same manner as in the section(i).

The measurement regarding the surface of the magnetic layer of thesample for measurement on the silicon wafer was performed using theellipsometer, and the refractive indexes of the magnetic layer (therefractive index Nx in a longitudinal direction, the refractive index Nyin a width direction, the refractive index Nz₁ in a thickness directionmeasured by incidence of incidence ray in the longitudinal direction,and the refractive index Nz₂ in a thickness direction measured byincidence of incidence ray in the width direction) were obtained by themethod described above by spectral ellipsometry. Nxy and Nz wereobtained from the obtained values, and the absolute value ΔN of thedifference of these values was obtained. Regarding all of magnetic tapesof the examples and the comparative examples, the obtained Nxy was avalue greater than Nz (that is, Nxy>Nz).

Vertical Squareness Ratio (SQ)

A vertical squareness ratio of the magnetic tape is a squareness ratiomeasured regarding the magnetic tape in a vertical direction. The“vertical direction” described regarding the squareness ratio is adirection orthogonal to the surface of the magnetic layer. Regardingeach magnetic tape of the examples and the comparative examples, thevertical squareness ratio was obtained by sweeping an external magneticfield in the magnetic tape at a measurement temperature of 23° C.±1° C.using an vibrating sample magnetometer (manufactured by Toei IndustryCo., Ltd.) under conditions of a maximum external magnetic field of 1194kA/m (15 kOe) and a scan speed of 4.8 kA/m/sec (60 Oe/sec). Themeasurement value is a value after diamagnetic field correction, and isobtained as a value obtained by subtracting magnetization of a sampleprobe of the vibrating sample magnetometer as background noise. In oneaspect, the vertical squareness ratio of the magnetic tape is preferably0.60 to 1.00 and more preferably 0.65 to 1.00. In addition, in oneaspect, the vertical squareness ratio of the magnetic tape can be, forexample, equal to or smaller than 0.90, equal to or smaller than 0.85,or equal to or smaller than 0.80, and can also be greater than thesevalues.

Missing Pulse Generation Frequency in Low Temperature and High HumidityEnvironment

The following measurement was performed in the low temperature and highhumidity environment of a temperature of 13° C. and relative humidity of80%.

A magnetic tape cartridge accommodating each magnetic tape (magnetictape total length of 500 m) of the examples and the comparative exampleswas set in a drive of Linear Tape-Open Generation 6 (LTO-G6)manufactured by IBM, and the magnetic tape was subjected toreciprocating running 1,500 times at tension of 0.6 N and a runningspeed of 8 m/sec.

The magnetic tape cartridge after the running was set in a referencedrive (LTO-G6 drive manufactured by IBM), and the magnetic tape isallowed to run to perform the recording and reproducing. A reproducingsignal during the running was introduced to an external analog/digital(AD) conversion device. A signal having a reproducing signal amplitudewhich is decreased by 70% or more than an average (average of measuredvalues at each track) was set as a missing pulse, a generation frequency(number of times of the generation) thereof was divided by the totallength of the magnetic tape to obtain a missing pulse generationfrequency (unit: times/m) per unit length (per 1 m) of the magnetictape. In a case where the missing pulse generation frequency is equal toor smaller than 5 times/m, the magnetic tape can be determined as amagnetic tape having high reliability in practice.

The results of the above evaluation are shown in Table 1 (Table 1-1 toTable 1-4).

TABLE 1-1 Example 1 Example 2 Example 3 Example 4 Magnetic liquid beaddispersion time 50 hours 50 hours 50 hours 50 hours Magnetic liquiddispersion bead 0.1 mm 0.1 mm 0.1 mm 0.1 mm diameter Magnetic liquid 330eq/ton 330 eq/ton 330 eq/ton 330 eq/ton Content of SO₃Na group ofpolyurethane resin Magnetic liquid 15.0 parts 15.0 parts 15.0 parts 15.0parts Content of SO₃Na group-containing polyurethane resin Non-magneticlayer forming 24 hours 24 hours 24 hours 24 hours composition dispersiontime Magnetic layer drying temperature  50° C.  50° C.  50° C.  50° C.Calender temperature 100° C. 100° C. 100° C. 100° C. Formation andalignment of magnetic Homeotropic alignment Second layer: homeotropicSecond layer: homeotropic Homeotropic layer 0.5 T alignment 0.5 T/alignment 0.5 T/ alignment 0.2 T First layer: no alignment First layer:no alignment process process Cooling zone staying time 1 second 1 second180 seconds 1 second Burnishing treatment before curing PerformedPerformed Performed Performed process Result Vertical squareness ratio0.66 0.60 0.60 0.60 (SQ) ΔN 0.30 0.35 0.35 0.25 Logarithmic decrement of0.048 0.048 0.015 0.048 magnetic layer Missing pulse generation 4 2 1 4frequency (times/m)

TABLE 1-2 Comparative Example 1 Comparative Example 2 ComparativeExample 3 Magnetic liquid bead dispersion time 6 hours 50 hours 6 hoursMagnetic liquid dispersion bead diameter 1 mm 0.1 mm 1 mm Magneticliquid 60 eq/ton 330 eq/ton 60 eq/ton Content of SO₃Na group ofpolyurethane resin Magnetic liquid 25.0 parts 15.0 parts 25.0 partsContent of SO₃Na group-containing polyurethane resin Non-magnetic layerforming composition dispersion time 3 hours 24 hours 3 hours Magneticlayer drying temperature 70° C.  50° C. 70° C. Calender temperature 80°C. 100° C. 80° C. Formation and alignment of magnetic layer No alignmentprocess Homeotropic alignment Homeotropic alignment 0.5 T 0.5 T Coolingzone staying time Not performed Not performed Not performed Burnishingtreatment before curing process Not performed Not performed Notperformed Result Vertical squareness ratio (SQ) 0.50 0.66 0.55 ΔN 0.100.30 0.20 Logarithmic decrement of magnetic layer 0.060 0.060 0.060Missing pulse generation frequency 10 7 9 (times/m)

TABLE 1-3 Comparative Example 4 Comparative Example 5 ComparativeExample 6 Magnetic liquid bead dispersion time 6 hours 50 hours 96 hoursMagnetic liquid dispersion bead diameter 1 mm 0.1 mm 0.1 mm Magneticliquid 60 eq/ton 330 eq/ton 330 eq/ton Content of SO₃Na group ofpolyurethane resin Magnetic liquid 25.0 parts 15.0 parts 10.0 partsContent of SO₃Na group-containing polyurethane resin Non-magnetic layerforming composition dispersion time 3 hours 24 hours 48 hours Magneticlayer drying temperature 70° C.  50° C.  30° C. Calender temperature 80°C. 100° C. 110° C. Formation and alignment of magnetic layer Homeotropicalignment No alignment process Homeotropic alignment 0.5 T 0.5 T Coolingzone staying time 1 second Not performed 1 second Burnishing treatmentbefore curing process Performed Not performed Performed Result Verticalsquareness ratio (SQ) 0.55 0.53 0.80 ΔN 0.20 0.20 0.45 Logarithmicdecrement of magnetic layer 0.048 0.060 0.048 Missing pulse generationfrequency (times/m) 7 10 8

TABLE 1-4 Comparative Example 7 Comparative Example 8 ComparativeExample 9 Magnetic liquid bead dispersion time 50 hours 96 hours 6 hoursMagnetic liquid dispersion bead diameter 0.1 mm 0.1 mm 1 mm Magneticliquid 330 eq/ton 330 eq/ton 60 eq/ton Content of SO₃Na group ofpolyurethane resin Magnetic liquid 15.0 parts 10.0 parts 25.0 partsContent of SO₃Na group-containing polyurethane resin Non-magnetic layerforming composition dispersion time 24 hours 48 hours 3 hours Magneticlayer drying temperature  50° C.  30° C. 70° C. Calender temperature100° C. 110° C. 80° C. Formation and alignment of magnetic layer Secondlayer: no Second layer: no Second layer: alignment process/ alignmentprocess/ homeotropic alignment First layer: homeotropic First layer:homeotropic 0.5 T/ alignment 0.5 T alignment 0.5 T First layer: noalignment process Cooling zone staying time Not performed Not performedNot performed Burnishing treatment before curing process Not performedNot performed Not performed Result Vertical squareness ratio (SQ) 0.600.66 0.53 ΔN 0.20 0.20 0.20 Logarithmic decrement of magnetic layer0.060 0.060 0.060 Missing pulse generation frequency (times/m) 11 9 11

From the results shown in Table 1, in the magnetic tapes of Examples 1to 4 in which ΔN and the logarithmic decrement of the magnetic layer arerespectively in the range described above, it is possible to confirmthat the missing pulse generation frequency in the low temperature andhigh humidity environment is decreased, compared to the magnetic tapesof Comparative Examples 1 to 9.

In general, the squareness ratio is known as an index for a state of theferromagnetic powder present in the magnetic layer. However, as shown inTable 1, even in a case of the magnetic tapes having the same verticalsquareness ratios, ΔN are different from each other (for example,Example 1 and Comparative Example 8). The inventors have thought thatthis shows that ΔN is a value which is affected by other factors, inaddition to the state of the ferromagnetic powder present in themagnetic layer.

One aspect of the invention is effective in a technical field of variousmagnetic recording media such as a magnetic tape for data storage.

What is claimed is:
 1. A magnetic tape comprising: a non-magneticsupport; a magnetic layer including ferromagnetic powder and a bindingagent on the non-magnetic support, and a non-magnetic layer including anon-magnetic powder and a binding agent between the non-magnetic supportand the magnetic layer, wherein the thickness of the non-magnetic layeris 0.1 to 1.5 μm, the absolute value ΔN of the difference between therefractive index Nxy measured regarding an in-plane direction of themagnetic layer and the refractive index Nz measured regarding athickness direction of the magnetic layer is 0.25 to 0.40, thelogarithmic decrement acquired by a pendulum viscoelasticity testperformed regarding the surface of the magnetic layer is equal to orsmaller than 0.050, and the logarithmic decrement on the magnetic layerside is determined by the following method: securing a measurementsample of the magnetic tape with the measurement surface, which is thesurface on the magnetic layer side, facing upward on a substrate in apendulum viscoelasticity tester; disposing a columnar cylinder edgewhich is 4 mm in diameter and equipped with a pendulum 13 g in weight onthe measurement surface of the measurement sample such that the longaxis direction of the columnar cylinder edge runs parallel to thelongitudinal direction of the measurement sample; raising the surfacetemperature of the substrate on which the measurement sample has beenpositioned at a rate of less than or equal to 5° C./min up to 80° C.;inducing initial oscillation of the pendulum; monitoring thedisplacement of the pendulum while it is oscillating to obtain adisplacement-time curve for a measurement interval of greater than orequal to 10 minutes; and obtaining the logarithmic decrement Δ from thefollowing equation:$\Delta = \frac{{\ln( \frac{A_{1}}{A_{2}} )} + {\ln( \frac{A_{2}}{A_{3}} )} + {\ldots{}{\ln( \frac{A_{n}}{A_{n + 1}} )}}}{n}$wherein the interval from one minimum displacement to the next minimumdisplacement is adopted as one wave period; the number of wavescontained in the displacement-time curve during one measurement intervalis denoted by n, the difference between the minimum displacement and themaximum displacement of the n^(th) wave is denoted by An, and thelogarithmic decrement is calculated using the difference between thenext minimum displacement and maximum displacement of the n^(th) wave(A_(n+1) in the above equation).
 2. The magnetic tape according to claim1, wherein the difference Nxy−Nz between the refractive index Nxy andthe refractive index Nz is 0.25 to 0.40.
 3. The magnetic tape accordingto claim 1, wherein the logarithmic decrement is 0.010 to 0.050.
 4. Themagnetic tape according to claim 1, wherein the thickness of thenon-magnetic layer is 0.1 to 1.0 μm.
 5. The magnetic tape according toclaim 1, wherein the thickness of the non-magnetic layer is 0.1 to 0.7μm.
 6. The magnetic tape according to claim 1, further comprising: aback coating layer including a non-magnetic powder and a binding agenton a surface of the non-magnetic support opposite to a surface providedwith the magnetic layer.
 7. A magnetic recording and reproducing devicecomprising: the magnetic tape according to claim 1; and a magnetic head.